Throttle-Body Hygiene
Q: I recently replaced the EGR valve in my 6.0-liter V-8-powered 2000 GMC Sierra 2500 because at idle it would stumble, randomly, and there was a lot of hesitation on acceleration as if the engine was choking. Since I replaced the valve, the hesitation is gone, but I'm still getting erratic idle at random, when the engine is warm. There aren't any engine lights or codes. Could this be the engine coolant-temperature sensor?
A: We can't give you a concrete answer, but here are two areas to focus on. Number one is cleaning out the throttle-body assembly. Carbon builds up on the back of the throttle plate and within the throttle bore. These accumulations come from the EGR gases emitted into the engine and can become a problem as mileage increases. The carbon buildup limits the air intake needed for quality idle control. Fixing it is a simple job you can do yourself. With the engine off, remove the rubber intake duct from the throttle-body assembly, open the throttle plate, and reach in with a rag saturated in GM Top Engine Cleaner. Keep wiping until almost all the accumulation is gone. You can even use a toothbrush to get into the nooks and crannies. It's also a good idea to bring the truck into a GMC service department to have the PCM updated with the latest calibration. This may help the computer compensate for future throttle-body accumulation and also deal with noted tip-in hesitations. Another component that may be responsible for an intermittent rough-idle condition or misfire are the fuel injectors. Injectors can become obstructed by fuel contaminants. A pressure-drop test of the injectors, with the use of special tools, can determine which injectors are obstructed, which need to be replaced, and which may be saved with the help of a direct injector-cleaning procedure. The special cleaning tool uses the same GM Top Engine Cleaner, which is good stuff. There also have been problems with corrosion at the fuel-injector electrical terminals causing intermittent driveability concerns.
Fighting off the Sludge
Q: My 1989 Bronco II has the six-cylinder 2.9-liter engine and a five-speed manual transmission. After sitting all night, the truck takes three or four tries before it'll remain running. It starts and runs for about a second or two and then dies. Once it stays running, it's fine the rest of the day. I checked the fuel pressure, and it's good according to the book: 40 psi when the key is turned on and about 34 psi while it's running. It also holds 40 psi for quite a while if I let it sit when it's not running. If this problem is the idle bypass valve, could you tell me how to check it, or give me some other troubleshooting ideas? No malfunction lights are on.
A: Like many other throttle body and port fuel-injected engines, carbon buildup on the throttle bore and plate or idle air control (or bypass) valve and passages can cause issues with idle quality and stalling. Cleaning out the carbon often becomes an unavoidable but relatively infrequent service procedure. More recently developed air-intake systems are designed to minimize or even eliminate the conditions associated with carbon buildup. In your case, along with a bunch of other 1984 to 1992 model-year Fords, there's an alternative fix to the carbon problem (or sludge, as Ford calls it). Ford developed a service kit with directions on cleaning the air-intake components, a detailed adjustment procedure, and the addition of a spacer plate beneath the idle air-control valve intended to permanently eliminate idle symptoms due to the sludge accumulation. Service kit 9F939 should fit your 1989 Bronco II, and instructions are included. As always, the source of the condition should first be confirmed and other possible causes of stalling eliminated before buying any parts. But if sludge is the culprit, you have a choice between repeated cleaning as maintenance, or a kit to install as a one-time repair.